[Chap. VII CELLS AS BIOLOGICAL UNITS 63 



hydrate, fat, and lipoid phases dispersed in water, in which are various 

 salts, sugars, acids, and other soluble compounds. Many of these sub- 

 stances are chemically unstable and react with extreme readiness. All 

 masses of protoplasm have a surface film rich in lipoids which prevents 

 unlimited dispersion and keeps the mass intact. 



The highly reactive character of the protoplasmic system at ordinary 

 temperatures is one of its most important properties. Since it is a col- 

 loidal system there are verv large surfaces between the water and the 

 other phases, and here surface energy may bring about reactions not 

 possible in a solution. These surfaces also accumulate electrical charges 

 which may be significant in reactions. Because it is a manv-phase svs- 

 tem, a change of one phase may result in the alteration of other inter- 

 locking phases and the modification of the whole protoplasmic system. 

 Like gelatin and jelly, protoplasm is more or less elastic, and some of its 

 constituents may have a brush-heap structure. 



Furthermore, protoplasm is a self-perpetuating system, in that it can 

 combine foods (carbohydrates, fats, and proteins) into the system and 

 enlarge its mass. It is evident, therefore, that the jellv-like part of the 

 cell which was named protoplasm is not a single chemical substance. 

 Life, like all the other distinctive properties of protoplasm, is a result of 

 the chemical components and their complex organization in a colloidal 

 system. When the system is disorganized the distinctive properties disap- 

 pear, for the materials or compounds are not alive. 



Cytoplasm. The protoplasm in most plant cells is organized into 

 cytoplasm, plastids, and nucleus. In young cells these parts completely 

 occupy the space within the cell wall ( Fig. 31 ) . As the cell enlarges, 

 microscopic droplets of liquid become visible in the cytoplasm, the col- 

 loidal matrix is drawn together, and the droplets coalesce, forming larger 

 water-filled cavities called vacuoles. In most plant cells the coalescence 

 of small vacuoles results finallv in the formation of a single vacuole, and 

 the cvtoplasm with its embedded plastids and nucleus becomes relegated 

 to a thin layer lining the cell wall. In other cells the nucleus may retain 

 its central position, supported by cvtoplasmic strands which are the 

 remnants of the cytoplasm separating the several vacuoles before they 

 coalesced into the single large one. 



Cytoplasm is usually granular in appearance because of the presence 

 of particles of various foods and other substances. Cvtoplasmic move- 

 ments (streaming) mav be seen in the cells of some plants. 



Plastids are protoplasmic bodies in the cytoplasm. Special reactions 



